Rapa Nui calendar

The Rapa Nui calendar was the indigenous lunisolar calendar of Easter Island. It is now obsolete.

William J. Thomson, paymaster on the USS Mohican, spent twelve days on Easter Island from December 19 to 30, 1886. Among the data Thomson collected were the names of the nights of the lunar month and of the months of the year:^[1]

The natives reckoned their time, and in fact do so still by moons or months, commencing the year with August, which was, according to the traditions,the time when Hotu-Matua and his followers landed upon the island.

Thomson recorded the months as follows:

Rapanui name	Meaning	Western equivalent, 1886–1887
Anekena		August
Hora-iti	little summer	September
Hora-nui	big summer	October
Tangarouri		part of November
Kotuti		November and December
Ruti		December and January
Koro		January
Tuaharo		February
Tetuupu		March
Tarahao		April
Vaitu-nui	big winter	May
Vaitu-poto	short winter	June
Maro or Temaro		July

The month was divided in two, beginning with the new and full moon. Thomson recorded the calendar at the time of his visit to the island as follows. The new moon occurred on November 25 and again on the night of December 24;^[2] Thompson records the crescent was first visible on November 26.

Rapanui name	Meaning	Western equivalent, 1886
Kokore tahi	first kokore	November 27
Kokore rua	second kokore	November 28
Kokore toru	third kokore	November 29
Kokore hâ	fourth kokore	November 30
Kokore rima	fifth kokore	December 1
Kokore ono	sixth kokore	December 2
Maharu	first quarter	December 3
Ohua		December 4
Otua		December 5
Ohotu		December 6
Maure		December 7
Ina-ira		December 8
Rakau		December 9
Omotohi	full moon	December 10
Kokore tahi	first kokore	December 11
Kokore rua	second kokore	December 12
Kokore toru	third kokore	December 13
Kokore hâ	fourth kokore	December 14
Kokore rima	fifth kokore	December 15
Tapume		December 16
Matua		December 17
Orongo	first quarter [sic]	December 18
Orongo taane		December 19
Mauri nui		December 20
Marui [sic] kero		December 21
Omutu		December 22
Tueo		December 23
Oata	new moon	December 24
Oari		December 25
Kokore tahi	first kokore	December 26

Three sources correspond with each other except for two intercalary days (in bold), and the night of the new moon in Englert, which seems to have been confused with one of these. Beginning with (o)ata, the night of the new moon, they are:

*New moon, full moon, and first and last quarters.

The kokore are unnamed (though numbered) nights; tahi, rua, toru, haa, rima, ono are the numerals 1–6. The word kokore is cognate with Hawaiian ‘a‘ole "no" and Maori kahore "no" and Tahitian ‘aore "there is/are not"; here it may mean "without [a name], nameless".[1]

The calendar collected by Thomson is notable in that it contains thirteen months. All other authors mention only twelve, and Métraux and Barthel find fault with Thomson:

Thomson translates Anakena as August and suggests that the year began at that time because Hotu-Matua landed at Anakena in that month, but my informants and Roussel (1869) give Anakena as July.^[3]

We are basing the substitution on the lists by Metraux and Englert (ME:51; HM:310), which are in agreement. Thomson's list is off by one month.^[4]

However, Guy^[5] calculated the dates of the new moon for years 1885 to 1887 and showed that Thomson's list fit the phases of the moon for 1886. He concluded that the ancient Rapanui used a lunisolar calendar with kotuti its embolismic month (AKA "leap month"), and that Thomson chanced to land on Easter Island in a year with a leap month.

The days hotu and hiro appear to be intercalary. A 28-day calendar month needs one to two intercalary days to keep in phase with the 29½-day lunar month. One of the rongorongo tablets may describe a rule for when to add these days.^[6]